Add a drop shadow cast by the frame

New plugin:hyprchrome:shadow_size (px, 0 = off), shadow_color and
shadow_offset (vec2). Off by default, so existing setups are unchanged.

The silhouette is the frame's own outer boundary - extracted out of
GetBorderTexture into AppendFrameOuterPath so the shadow and the shape
casting it can't drift apart - filled into an A8 mask, blurred with three
box passes, then tinted. The frame's cutouts (side notches, the strip
beside the title bar) are part of that outline, so they cast their shape
for free.

Three things worth not undoing later:

- The window's interior is cleared after the blur, not before. Punching
  it first leaves the blur smearing shadow inward across the window's own
  content. Clearing afterwards puts the cut exactly on the ring's inner
  boundary, where the frame's opaque pixels hide it.
- The shadow is not part of the decoration's reserved extents, since
  reserving it would push neighbouring windows away by the shadow's
  width. It's drawn past the decoration's box instead, which is why
  damageEntire() and boundingBox() expand by ShadowMarginLogical().
- It renders at no more than kShadowMaxDim px on the long edge and lets
  the GPU upscale. A blurred blob loses nothing to that, and a
  full-resolution blur would otherwise be re-run per frame for the whole
  length of a resize animation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -61,6 +61,9 @@ Config values live under `plugin:hyprchrome:*` and are declared in `src/GlobalSt
 - `active_color` / `inactive_color` (gradient — one or more colors + optional angle, same syntax as `general:col.active_border`) — used when `follow_hyprland_border_color` is false, or when Hyprland reports no border color at all
 - `glow_size` (int, px the border's color bleeds inward past the window edge, over the window itself — 0, the default, disables it)
 - `glow_strength` (float 0–1, peak opacity of that glow at the window edge, as a fraction of the border color's own alpha)
+- `shadow_size` (int, px the frame's drop shadow reaches past its own outline — 0, the default, disables it)
+- `shadow_color` (color, its alpha being the shadow's opacity)
+- `shadow_offset` (vec2, px the shadow is displaced by; positive is right/down)
 - `titlebar_height` (int, px)
 - `titlebar_text_size` (int, px)
 - `titlebar_font` (string, passed to `cairo_select_font_face`)
@@ -80,7 +83,8 @@ Per-window decoration (`src/ChromeDecoration.hpp/.cpp`): implements `IHyprWindow
 - `getPositioningInfo()` reserves screen space around the window (`DECORATION_POSITION_ABSOLUTE`, all four edges) sized by `extent` (+ `titlebarHeight` on top) — this is what makes Hyprland leave room for the border/title bar instead of it overlapping neighboring windows.
 - `draw()` doesn't render directly; it enqueues a `ChromePassElement` into Hyprland's render pass each frame.
 - `GetBorderTexture(...)` and `GetTitleTexture(...)` are the actual cairo/Pango rendering entry points, each memoizing against their own last-seen parameters (size, extent, chamfer, color, title text, font, etc.) so unchanged frames reuse the cached `Render::ITexture` instead of re-rendering.
-- `FullDecorationExtentGlobal()` computes the decoration's box in global logical coordinates, accounting for workspace animation offset and floating-window offset.
+- `FullDecorationExtentGlobal()` computes the decoration's box in global logical coordinates, accounting for workspace animation offset and floating-window offset. It deliberately excludes the drop shadow (below), which is drawn outside it.
+- `GetShadowTexture(...)` renders the drop shadow. The silhouette is `AppendFrameOuterPath` — the same outline `GetBorderTexture` fills, extracted specifically so the two can't drift apart — filled solid into an A8 mask, blurred, then tinted. Three things about it are load-bearing: (1) the window's interior is cleared *after* the blur, not before, since punching it first would smear shadow inward across the window's own content; the cut lands exactly on the ring's inner boundary so the frame's opaque pixels hide it. (2) The shadow is not part of `getPositioningInfo`'s reserved extents — reserving it would push neighbouring windows away by the shadow's width — so it's simply drawn past the decoration's box, which is why `damageEntire()` and `boundingBox()` have to expand by `ShadowMarginLogical()` by hand. (3) It's rendered at most `kShadowMaxDim` px on the long edge and upscaled by the GPU; a blurred blob loses nothing to that, and it caps a cost that would otherwise be paid per frame of a resize animation. The blur itself is three box passes (`BlurA8Surface`), transposing between each so the vertical pass reuses the horizontal one's cache-friendly row code.
 - The inward glow (`glow_size`/`glow_strength`) is drawn by `DrawInwardGlow` into the hole the ring's even-odd fill leaves behind, so it lands on the window's own pixels (this decoration is `DECORATION_LAYER_OVER`, and the texture spans the whole window box, not just the ring). Its falloff is built from *overlapping* fills (one layer per px of depth, clamped to `kGlowMinLayers`/`kGlowMaxLayers`) — layer *i* covers the window edge inward to depth `glowPx * i / layers`, so a pixel `d` from the edge is painted by every layer deeper than `d`. The profile is stated explicitly (`strength * (1 - d/glowPx)^kGlowFalloffExponent` — fast off the edge, easing into a tail that reaches zero tangentially so there's no ring where it stops), and since each layer composites over every deeper one, a layer's own alpha is *not* its target: it's solved outermost-inward as `1 - a_j = (1 - T_j) / (1 - T_j+1)`. Changing the profile means changing `targetAt`, not the per-layer alphas. Abutting disjoint bands instead would leave an antialiasing seam at every shared edge; that's the reason for the overlap, don't "optimize" it away. Every layer's *outer* edge is the ring's inner boundary verbatim, chamfer vertices and all — filleting or otherwise altering it detaches the glow from the frame and opens a sliver of unpainted window at each corner. The corner softening lives entirely on the layers' *inner* edges, which are what the accumulated falloff's contours actually follow: each is inset by its own depth, filleted by `kGlowCornerSmoothing` × that depth (`AppendFilletedPolygon`, a quadratic Bezier through each vertex), and has its chamfer shrunk by `kChamferInsetShrink` × that depth. That last correction is not optional cosmetics — insetting a chamfered rect while holding its chamfer constant moves the 45° face in by `d·√2` rather than `d`, so without it the glow runs ~41% deeper at every corner than along the sides. The per-layer alpha is baked into the gradient pattern (`CreateGradientPattern`'s `alphaScale`) specifically so each layer can be a `cairo_fill` of its own band rather than a clip + `cairo_paint_with_alpha`, which would rasterize the clip's full extents — i.e. the whole window area — once per layer.
 
 Render-pass element (`src/ChromePassElement.hpp/.cpp`): an `EK_CUSTOM` pass element (`ChromePassElement::draw()`) that runs once per frame per window and: